Weird lathe bed

[RainbowsParticipant @rainbows]

Looking at the artisan lathe on the internet I was trying to work out how the bed works.

 

 

I am used to dovetail way lathes having a gib that mates one of the dovetails. The artisan instead has a gib that mates on a flat.

To my understanding that would mean that as wear occurs the rear dovetail would get pushed out of engagement. Thought the back dovetail bolted onto the saddle I'm not sure if that is for adjustment during use or just to make construction of the lathe cheaper.

 

I don't own one of these lathes this is just purely out of curiosity. Anyone good at machine design theory understand a reason for the weird gib placement? 

Edited By Rainbows on 13/09/2016 14:26:53

[RainbowsParticipant @rainbows]

[Michael GilliganParticipant @michaelgilligan61133]

… Very Interesting

I agree with your analysis … It looks like the 'bearing' is actually narrow, and the bolt-on gib at the rear is just 'anti-lift'.

We had a broadly similar discussion about the plate under the bed of Brian John's little Optimum lathe.

MichaelG.

[Ady1Participant @ady1]

This is the Drummond M series bed system.

Perhaps it was easier to put the tapered gib system within the bed because there was too much gubbins on the outside of the apron

[Roderick JenkinsParticipant @roderickjenkins93242]

Posted by Michael Gilligan on 13/09/2016 14:34:45:

… It looks like the 'bearing' is actually narrow, and the bolt-on gib at the rear is just 'anti-lift'.

Agreed. The narrow guide was much favoured in lathe design and I read somewhere that after Myford moved to the wide guide using both front and rear shears, Tubal Cain, when he bought a new Myford S7, modified it back to the narrow guide.

Cheers,

Rod

[Ady1Participant @ady1]

The cantilever bed on it is HUGE

**LINK**

[RainbowsParticipant @rainbows]

It some what sounds like the guy who designed it had only seen a lathe from afar.

Common sense to me would have told me that using both ends of the bed would give a more stable saddle compared to a narrow guide.

That said I finally know now that when people refer to Tubal Cain here it is Tom Walshaw and not mrpete222… took a while to figure that out.

[Neil WyattModerator @neilwyatt]

Posted by Roderick Jenkins on 13/09/2016 14:45:01:

Posted by Michael Gilligan on 13/09/2016 14:34:45:

… It looks like the 'bearing' is actually narrow, and the bolt-on gib at the rear is just 'anti-lift'.

Agreed. The narrow guide was much favoured in lathe design and I read somewhere that after Myford moved to the wide guide using both front and rear shears, Tubal Cain, when he bought a new Myford S7, modified it back to the narrow guide.

And J. A. Radford went the other way…

FWIW many lathes of that size these days use an inverted-v bed that effectively gives a narrow guide at the front without the need for gibs, just adjustable plates front and back to stop the saddle lifting.

Neil

[Roderick JenkinsParticipant @roderickjenkins93242]

J A Radford was correcting wear in his lathe and wanted to utilise the un-used rear vertical shear. An argument against using only the front shears is that this is shared with the tailstock and therefore it is prone to more wear. The argument for the narrow guide is that for the same amount of play you get less rotation of the saddle than you would with a wide guide (I think!). By having both shears and the leadscrew all grouped close together there is less freedom of movement. I don't know why Myford changed though – was it just the wear issue?

Rod

[BazyleParticipant @bazyle]

With a narrow guide you need the gib screws at the front which compromises the apron design. Rear shear gib is easier to access and adjust on a factory line.

[Neil WyattModerator @neilwyatt]

Posted by Roderick Jenkins on 13/09/2016 17:35:43:

J A Radford was correcting wear in his lathe and wanted to utilise the un-used rear vertical shear. An argument against using only the front shears is that this is shared with the tailstock and therefore it is prone to more wear. The argument for the narrow guide is that for the same amount of play you get less rotation of the saddle than you would with a wide guide (I think!). By having both shears and the leadscrew all grouped close together there is less freedom of movement. I don't know why Myford changed though – was it just the wear issue?

Rod

I'm sure Radford wrote he couldn't understand why Myford didn't use the rear shear, I think it may have been his experience that caused them to change, for better or worse.

Neil

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Roderick Jenkins on 13/09/2016 17:35:43:

The argument for the narrow guide is that for the same amount of play you get less rotation of the saddle than you would with a wide guide (I think!).

.

+1

… It's simple geometry

MichaelG.

[Neil WyattModerator @neilwyatt]

Posted by Michael Gilligan on 13/09/2016 19:02:22:

Posted by Roderick Jenkins on 13/09/2016 17:35:43:

The argument for the narrow guide is that for the same amount of play you get less rotation of the saddle than you would with a wide guide (I think!).

.

+1

… It's simple geometry

MichaelG.

Actually it is simple geometry, but you get the same amount of twist for any given amount of play regardless of width:

sin(twist angle) = amount of play/guide length

I can't see width in that equation.

The narrow guide theory states that it is less likely to jam if the distance between the guides is less than three times the length. But has anyone here ever experienced their lathe carriage jamming?

Neil

[HOWARDTParticipant @howardt]

With a design like the Artisan the taper gib allows fine adjustment by using a screw on the end to push and pull it into place. Much easier to adjust clearances than fiddling with grub screws on the opposite dovetail as we have to now on the small imports.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 13/09/2016 19:19:28:

…

sin(twist angle) = amount of play/guide length

I can't see width in that equation.

.

… Not only was I wrong, but it's even worse than you thought

The 'guide length' in your equation is the hypotenuse of a triangle, and is therefore longer for the wide guide.

MichaelG.

[RainbowsParticipant @rainbows]

Posted by Roderick Jenkins on 13/09/2016 14:45:01:

Posted by Michael Gilligan on 13/09/2016 14:34:45:

… It looks like the 'bearing' is actually narrow, and the bolt-on gib at the rear is just 'anti-lift'.

Agreed. The narrow guide was much favoured in lathe design and I read somewhere that after Myford moved to the wide guide using both front and rear shears, Tubal Cain, when he bought a new Myford S7, modified it back to the narrow guide.

Cheers,

Rod

Has any myford lathe had a narrow bearing? I thought they all had the same lay out.

[AjohnwParticipant @ajohnw51620]

Michael G is more or less correct – it's length that sets the twist so a narrow guide needs to be long in relationship to it's width. It's one of the advantages of prismatic beds with an inverted V on top as guide.

The not so narrow guide often occurs in dovetail beds. Sometimes the saddle is scarcely wider than the width of the bed = not a good idea at all even from a wear point of view.

The Artisan is weird. The only way that can work well is if the gib causes the rear dovetail to be in good contact with the saddle. The front V isn't going to do anything but that probably doesn't matter providing cutting forces push the saddle down and against the gib / rear dovetail.

Doesn't make much sense. Might be drawn reversed and the bolt on bit should be at the front. The sort of unused dovetail half might be of use if the lathe was used for milling. Some don't like that as the saddle isn't held down.

The cantilever wont matter providing it;s all rigid enough. Don't ask me what make but I did use a lathe that just had a pillar under the head stock end that contained the motor and drive. There were 2 of them. Swing probably about 12" maybe a bit more. They were decent lathes but looked rather odd. For size the centre distance was under similar sized lathes. Probably about 24".

John

–

Edited By Ajohnw on 13/09/2016 21:12:18

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Rainbows on 13/09/2016 20:33:57:

Has any myford lathe had a narrow bearing? I thought they all had the same lay out.

.

Pre-1972, narrow

Post-1972, wide

**LINK**

http://modelengineeringwebsite.com/Myford_era.html

MichaelG.

[blowlampParticipant @blowlamp]

Posted by Neil Wyatt on 13/09/2016 19:19:28:

Posted by Michael Gilligan on 13/09/2016 19:02:22:

Posted by Roderick Jenkins on 13/09/2016 17:35:43:

The argument for the narrow guide is that for the same amount of play you get less rotation of the saddle than you would with a wide guide (I think!).

.

+1

… It's simple geometry

MichaelG.

Actually it is simple geometry, but you get the same amount of twist for any given amount of play regardless of width:

sin(twist angle) = amount of play/guide length

I can't see width in that equation.

The narrow guide theory states that it is less likely to jam if the distance between the guides is less than three times the length. But has anyone here ever experienced their lathe carriage jamming?

Neil

Can someone explain this to me because Roderick Jenkins' post seems to be correct?
Or have I missed something

Martin.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by blowlamp on 13/09/2016 22:03:38:

Can someone explain this to me because Roderick Jenkins' post seems to be correct?
Or have I missed something

Martin.

.

If the [Z axis] spacing between the bearings is actually longer, then there will be less twist for a given clearance … But if it is only 'proportionately' longer then [as Neil asserted] it makes no difference.

In practice [as I responded] the effective spacing is lengthened with the wide [X axis] arrangemet.

… Get the sketch-pad, and a glass of Malt.

MichaelG.

[RainbowsParticipant @rainbows]

So

twistiness factor = width of saddle (z axis) / dept of saddle (x axis) ??

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Rainbows on 13/09/2016 22:48:56:

So

twistiness factor = width of saddle (z axis) / dept of saddle (x axis) ??

.

Not if Neil's logic is correct

… Actual twist will depend solely upon the [Z axis] length [i.e. your 'width'] of the saddle, and the clearance in the bearing arrangement.

Note: This may become rather more complicated if the [Z axis] length of the bearing surface differs at the various locations.

MichaelG.

.

P.S. Here's some bedtime reading:

https://archive.org/details/lathebeddesign00hornrich

Edited By Michael Gilligan on 13/09/2016 23:19:10

[Roderick JenkinsParticipant @roderickjenkins93242]

From Tony's site:

"One unusual aspect of the lathe's design (shared with the ML7) was the arrangement of the saddle; although this had equal-length wings at front and back, at the front (where the thrust was taken out on the inside vertical way) only half the length bore against the bed.
However, from August 1972 and Serial No. SK 108891B, the thrust was changed to bear against the full length of the saddle at the rear – the alteration being brought about by correspondence with an Australian engineer who had conducted the necessary practical experiments. This change also helped to ensure the success of the power cross-feed mechanism introduced in March 1974, from Serial No. SK 115830"

Presumably Tony meant NZ rather than Oz.. Because the inner side of the front saddle bearing is short to allow the tailstock to get closer to the chuck, the "narrow guide" does not have a particularly high aspect ratio so it is arguable that it is not a narrow guide at all. I'll see if I can find the Tubal Cain comment.

Here's the Radford article (page 2 is how he actually did the mod)

All very interesting, in theory at any rate. My S7 is one of the last "narrow guides" but I'm not sure I'm that bothered .

Rod

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Michael Gilligan on 13/09/2016 23:15:20:

Note: This may become rather more complicated if the [Z axis] length of the bearing surface differs at the various locations.

.

… As conveniently shown here: **LINK**

https://goo.gl/images/bXEiqt

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Roderick Jenkins on 13/09/2016 23:20:03:

Because the inner side of the front saddle bearing is short to allow the tailstock to get closer to the chuck, the "narrow guide" does not have a particularly high aspect ratio so it is arguable that it is not a narrow guide at all.

…

All very interesting, in theory at any rate. My S7 is one of the last "narrow guides" but I'm not sure I'm that bothered .

Rod

.

Nicely put, Rod

MichaelG.

[Author]

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